This thesis investigates the use of Tow-Based Discontinuous Composite (TBDC) interleaves to enhance the mode I fracture toughness of adhesively bonded joints with Carbon Fiber Reinforced Polymer (CFRP) substrates. Aiming to improve joint safety by slowing crack propagation and facilitating less sudden failure, this study focuses on integrating interlaminar-toughened substrates to resist crack growth and enhance the fracture behavior in the joint's substrate. Two main research questions are addressed: the influence of TBDC interleaves on mode I fracture toughness of CFRP laminates and their subsequent effect when used in CFRP-based bonded joints.

For CFRP laminates, Double-Cantilever Beam (DCB) samples were tested across three configurations: a non-toughened baseline and two TBDC-toughened variants. Based on previous research, three DCB configurations identified as the most promising for leveraging TBDC toughening in adhesive joints were tested. The [90/45/-45/TBDC/0]s and [90/60/90/-60/TBDC/0]s laminate substrates were bonded with the low-toughness adhesive Araldite 2015-1, while the [0/TBDC/90_2/0]s substrate was bonded with AF 163-2U, a high-toughness adhesive.

TBDC-toughened CFRP laminates demonstrated up to 130% higher fracture toughness compared to non-toughened counterparts. This was due to TBDC material crack propagation mechanisms such as crack branching, deflection, and fiber bridging.

In adhesively bonded joints, TBDC interleaves in CFRP substrates enhanced the decay of fracture toughness in specimens where cracks deflected from the bond line into the substrate, leading to a less abrupt reduction after reaching peak values. Joints with low-toughness adhesive exhibited more than a 100% increase in crack length from peak fracture toughness to the final value compared to non-TBDC-toughened substrate joints. Meanwhile, joints with high-toughness adhesive demonstrated toughness values 150% to 750% greater than those observed in non-toughened configurations at comparable crack lengths.

These findings highlight the potential of TBDC interleaves to enhance joint toughness, presenting new pathways to improve the safety of composite bonded structures.

The concept of lightweight design is driving future aircraft to exploit all the available strength of materials and further reduce the weight of an aircraft leading to lower fuel consumption and more sustainable aviation. Current designs introduce rivets and bolts to join the structure and simultaneously create holes in the pristine material, which is not wanted due to the local stress concentration. If the design wants to exploit all the strength of the material, stress concentration should be avoided using suitable joining technologies that don't require holes in the structure. This is possible with the adhesive bonding technology. However, many factors, such as the effects of manufacturing and impact-induced damage are still not fully understood. Thus, the damage tolerance of the design cannot be guaranteed. This results in conservatory safety factors being prescribed in the design process, which possibly reduces the exploitation of all available material strength.

The occurrence of barely visible impact damage (BVID) in aircraft composite structures, especially in adhesive joints, is a serious issue that can jeopardise an aircraft's structural safety during operation. However, virtual testing and numerous numerical approaches allow high-fidelity simulation of damage initiation and propagation in adhesives and composite adherends to predict the residual strength of the bonding accurately. Although the development is fast, accurate impact simulation is still computationally very expensive and thus not applicable in industrial cases where behaviour needs to be analysed to assess the structures' design, maintenance and repair.

This thesis investigated two topics that allow robust and accurate simulation methods to assess the effect of manufacturing and impact damage on the residual strength of bonded joints. The first topic is related to the development of the composite material damage model, where the multiscale material model is created by the use of a representative volume element (RVE). The second topic of simulation methodology is the development of the quasi-static simulation approach for the damage tolerance assessment of bonded joints. Modelling approaches to simulate residual strength are investigated through the finite element modelling of three groups (i) pristine, (ii) artificially damaged during manufacturing, and (iii) impacted bonded joints. In the numerical models, an approach based on the observation of the fracture surface of single-lap joints in different geometry and layup configuration is proposed in which the damage assessment focuses on the bondline area up to the first 0° ply. For the modelling of the damage in joints, two modelling techniques were studied, first removing elements in the damaged area and second detachment of the elements. Utilising those techniques, simplified approaches to model damage resulting from the impact were studied, with the modelling of impact damage as a hole, where all through-thickness elements are deleted and as delamination, where interlaminar cohesive zone elements are deleted.

The developed multiscale material model allowed for an accurate representation of failure modes occurring in the composite material by the characterisation of elastic, damage and plasticity parameters of the fibre and matrix constituents in the homogenisation and inverse characterisation process. The results showed that the deletion of the elements can be used to represent defects and damage of different kinds in the composite bonded joints, both in the adhesive and adherend. The comparison of different representations of the impact damage in the single lap joint configuration revealed that the best prediction in terms of the ultimate load, failure mode and size of the model is obtained with the simplified representation as a single delamination positioned before the first 0° ply in the layup and in the studied adherend configuration that was between the first (45°) and second (0°) ply in the layup. The study ends with the conclusion that in different geometries of single lap joints and different damage types studied, a numerical analysis should focus on the region of the overlap edge and in the thickness direction from the bondline up to the first 0° ply in the lay-up. The results and numerical method developed during this thesis create a base for the further investigation of a variety of impact cases on bonded joints.

Adhesively bonded joints have proven to outperform their mechanically fastened joint counterparts, as they present a more structurally efficient method of load transfer, lower stress concentrations and better fatigue performance at reduced weight. In the specific case of the Adhesively Bonded Single Lap Joint (ABSLJ), bending-induced stresses that result from the load path eccentricity add up to the adherend inplane stresses. Moreover, significant peak peel and shear stresses develop at the lap ends of the adhesive and associated adherend interlaminar tensile stresses have a detrimental effect on the joint’s strength. Such joints made of Fiber Reinforced Polymer (FRP) adherends bonded with an epoxy adhesive layer sustain a substantial amount of damage, from failure onset to ultimate failure. With the purpose of design structurally efficient and damage tolerant composite joints, it is essential to understand the stress distribution and to accurately predict the damage initiation and propagation events in such joints made of composite materials.

A well-established set of Damage Progression Models (DPMs) in the framework on the Continuum Damage Models (CDMs) were developed as a tool to predict the global response,damage initiation load and ultimate load of the specimens. Hashin 3D, Puck and LaRC05 werethe implemented failure criteria to detect the initiation of damage in the adherends. After thispoint, the elastic properties of the detected damage elements were reduced according to sudden and gradual material degradation models. As for the adhesive, the von Mises criterion was used to detect the damage onset and a linear softening law modeled the material degradation. For the validation of the DPMs, the numerical results were compared against the data of an already published experimental study. Four different adherend layup sequences: [45/90/ − 45/0]2푠, [90/−45/0/45]2푠, [0/45/90/−45]2푠 and [45/0/−45/0]2푠 were studied based on data extracted
from the mechanical testing, Digital Image Correlation (DIC) and Acoustic Emission (AE).

Good correlations between numerical predictions and averaged experimental linear stiffnesses were found, particularly for the two configurations with the outmost ply at 45∘, for which the difference was lower than 5%. The initial non-linear stage of the global response seems to be governed by the longitudinal bending stiffness, while the subsequent linear behavior is controlled by the longitudinal membrane stiffness of the adherends. Regarding damage initiation, numerical predictions showed to be 11.5%, 7.5%, 29.9% and 6.1%, respectively, more conservative for the four analysed configurations, when compared to the AE results, whose established criterion should be further developed. With respect to the ultimate load, the relative differences between predictions and tests showed significant variability among the tested configurations; specifically the deviations were of: 33.2%, 37.4%, -0.4% and -13.71%.

Despite the encouraging results, an inherent shortcoming of CDMs is the representation of damage in a smeared manner due to the homogenization of the anisotropic material in the modeling process. A blended framework using CDMs to model intralaminar failure and discrete crack models to model interlaminar failure and matrix cracking might lead to more realistic damage patterns.

Many movable bridges, built in the 1950’s and 1960’s, reach their end of service life or do not meet future traffic demands. Renovation of those bridge, if possible, is preferred over newly built from an economical point of view. Fibre-reinforced polymer (FRP) decks prove to be excellent for retrofitting bridge decks compared to traditional decks, mainly due to its high strength-to-weight ratio. Lightweight bridge decks are advantageous in movable bridges considering the savings on foundation, counter- weights and mechanical equipment, not to mention, reduced transportation costs, installation time and traffic hindrance during execution.
Connecting FRP decks to the steel girders can either be done mechanically (bolts), chemically (bonded) or in a hybrid fashion. Adhesively bonded connections do not require drilling in the FRP deck, thereby increasing its durability, have a more uniform stress distribution and fabrication costs are lower compared to bolted connections. Complex stress states and strength prediction of a bonded joints are yet not fully understood, therefore rarely being applied in primary load bearing structures like bridges.
This thesis focuses on the stress analysis and strength prediction of adhesively bonded connection between FRP decks and steel girders. An existing bridge, representative for renovation projects, is considered as case study. Its former bridge deck is replaced by a FRP deck and adhesively bonded to the steel girders. Structural analysis of the bridge deck performed with a global and local numerical model, focuses on the stress states in the bonded deck-to-girder connections. Traffic and thermal loads are the governing load cases, which show the largest stress concentrations. Peak stress levels are obtained along the edges and ends of the bonded connection between the FRP deck and secondary girders.
Comparison of results from the global and local model showed significant difference in stress levels and was further investigated. Stress concentration factors are determined to relate the (peak) stresses from the global and local model for different adhesive thickness and elastic modulus. Stress results from the global model are tweaked with the stress concentration factors and compared with strength values from literature. It can be concluded that bonded FRP/steel deck-to-girder connections are critical details in movable bridges.

This thesis takes an initial step in the research on the long term behaviour of composite-to-metal bonds that are loaded under hygrothermal conditions. It focuses on the response and behaviour of the adhesive in these bonds under moisture and loading. The thesis is inconclusive on the effect of loading on the water uptake. The load level and environment were however found to have a significant influence on the creep behaviour and residual strength of the bonds. It was suggested that at the initial stages of water absorption creep is suppressed while at later stages with higher moisture percentages creep is promoted. Low load levels had a positive effect on the residual lap shear strength and stiffness. For higher load levels and immersion in water the residual lap shear strength and stiffness are decreased. The experimental work also suggests that Araldite 2015 might be affected by ambient humidity.

Adhesive bonded joints are commonly used in structural applications. The thickness of the bondline has an important influence in the resistance of the joint. Information on this topic is mostly available for thin bondlines; up to 1 mm. However, little information is known about the effect of increasing the bondline thickness. This study consists of a literature study to analyze the trends on the mechanical properties such as shear strength, peel strength and fracture toughness due to increasing bondline thickness; an experimental part that studies the shear strength of double lap joints loaded in tension using one brittle and one ductile adhesive; and a numerical part that assesses whether the experimental results can be reproduced using Finite Element techniques. From the experiments, thickness dependency on multiple parameters such as the lap shear strength, the deformation capability and the strains at failure is proven. For both adhesives, the lap shear strength decreased with thickness. The Finite Element Analysis are able to accurately predict the failure initiation for the thick joints and the deformation capability up to yield for all the thicknesses.

The trailing edge of wind turbine blades are commonly manufactured as an adhesive joint of the pressure-side and suction-side composite panels of the blade. Under some conditions, a lead-to-trailing (LTT) edgewise bending moment can induce buckling at the trailing edge adhesive joint, which may lead to early failure of the blade due to delamination. As a structural instability, buckling in wind turbines has been the focus of much research especially in full-scale tests and more recently at the sub-component level. These higher-level tests, however, are done on pre-manufactured wind turbine blades and require extensive preparation in order to adapt the testing rig to each blade section, as well as incurring into elevated costs.
An additional test level has been suggested for elements and details of wind turbine blades. It has been suggested that this level can fulfill many purposes: New concepts, modifications, material combinations and orientations can be tested, partial safety factors of larger scale tests can be reduced or even certifying minor details of the blade can be done at the element and detail level. As such, the focus of this project is to develop a testing method for a simplified trailing edge bonded joint with a custom designed hinged clamping system upon which a compressive moment can be imposed to induce buckling.
The design of this test will initially be based on a semi-analytical buckling plate model, where in-plane and out-of-plane displacements are coupled through the Von Karman strain-displacement relations. This semi-analytical tool is employed to quickly estimate the buckling loads for plates of varying dimensions. Strain-free imperfections can be included in the model for twisted/pre-bent plates in order to estimate their effect on reducing the load-bearing capacity of the structure. The semi-analytical tool is complemented with FE models for all the design parameters.
The semi-analytical and numerical results are compared to demonstrate the agreement of both approaches aimed to provide a sturdy base for the research. Next, the experimental buckling loads and force-displacement curves are shown against the predictions from the previous approaches with good agreement. Nevertheless, the observable discrepancies between the experimental and numerical results showed that the desired joint-fixity at the boundaries was not fully realized, therefore leading to a slightly different post-buckling behavior. In the end, suggestions are given to improve on the experimental clamping system in order to improve and expand the scope of this research.

The use of fibre reinforced composite polymeric materials for primary helicopter structures has been increasing over the last decades due to their attractive properties; high performance and lightweight inducing more fuel savings than their metallic counterparts. With respect to the joining methods; structural adhesive bonding is preferred when cost and weight are important factors. The qualification of a bonded joint requires the determination of its durability and reliability of the bond strength, but due to the possible degradation of adhesives under certain environments, the lack of a failure criterion and in view of safety consideration, adhesive joints tends to be ‘overdesign’, resulting in an increase of weight and manufacturing costs.
The present project aims to evaluate and investigate the adhesive performance of carbon fibre reinforced PPS; a reinforced high performance thermoplastic qualified in Airbus and already used on certain structural parts of the helicopter. The surface topography and chemistry were evaluated using an optical microscope, XPS and contact angle measurements before and after two surface treatments; hand sanding and grinding. Adhesive joints were manufactured to analyse its single lap shear (SLS) strength and fracture energy, GIC using three different adhesives and two environmental conditions.
Both surface treatments increased the surface energy, but while sanded samples showed an increase of the dispersive component, APP increased to a large extent the polar part. In addition, after hand sanding the morphology of the surface was modified. For most of test configurations, the specimens failed at the interface, however plasma treated samples showed higher values of lap shear strength and fracture toughness energy.
An accelerated ageing condition was simulated by the storage of the samples in a climate chamber at high temperature and humidity. The effect that these environmental conditions had on the adhesion performance depended to a large extent on the adhesive used; in this case all of the adhesive studied showed different trends on the adhesion properties; the epoxy adhesive designed to withstand high temperatures did not present any effect on the adhesive performance but a common epoxy for structural application showed a softening of the adhesive and therefore a decrease on the shear strength but an increase of the fracture toughness and with respect to polyurethane adhesive, the interface was degraded changing the failure from mixed to adhesive mode.
Finally, CF-PEEK samples were tested in single lap shear after these two surface treatments, showing better results than PPS with the same working parameters. However, some configurations still failed at the interface.

This report details the design of a mission aimed to find and analyse active Venusian volcanoes, if they exist. These volcanoes are interesting because active volcanism would significantly contribute to the understanding of the Venusian atmosphere, its extreme climate and geological processes. This knowledge would in turn help us understand Earth better. The design is based on the concept selected previously in the Midterm report and consists of five vehicles: a spacecraft, an aeroshell, an aircraft and two landers. The spacecraft with aeroshell will be launched into a Hohmann transfer orbit to Venus in 2023. Upon arrival, the satellite will map the surface, and find the most promising region for volcanic activity. It will then deploy the aeroshell containing the aircraft and landers. The satellite then changes its orbit to one that allows for it to act as a relay between the Venusian vehicles and Earth. After entry and having slowed down sufficiently to deploy a parachute, the first lander will be dropped. This lander will act as a reference for the lander inside the aircraft. Next, the aircraft is deployed after which it will start following flight tracks that allow for it to stay in the Sunlight. These tracks are designed by taking into consideration the power systems, thermal system and propulsion system, and then optimising such that the electronics do not overheat and that the battery size is reasonable. While flying, the aircraft will take measurements to locate volcanoes. Once a very promising location is found, the aircraft will deploy the second lander from an altitude of about 32 km. This lander will then descend further down and land on the surface where it will perform measurements. Combining the measurements of all vehicles it is expected that the mission can also complete a number of secondary objectives to further improve the knowledge of Venus...